It is frequently desirable to measure the thickness of single pane glass plate products such as single panes of glass, mirror products, tinted glass, and bullet proof glass. It also is desirable to measure the thickness of individual panes of composite glass products such as laminated glass products, and to measure the glass thickness and glass spacings of multiple pane, sealed insulated glass windows. These measurements are desirable for the glass product manufacturer, for glass product purchasers, and for building inspectors to verify that glass products are within specifications.
One method for measuring the thickness of glass plates or panes is to place a mirror on one side of the pane and a gauge on the opposite side of the pane, as shown in U.S. Pat. 4,848,943. The gauge is positioned so that when the user looks down a sight tube, the reflection of a target on an adjustable scale is seen in the mirror. When the scale is adjusted to line the target with reference marks, the glass thickness is indicated on the scale. The gauge also can be used to measure the total thickness of multiple pane windows. If the glass thickness is known, the glass pane spacing can then be determined. However, it cannot measure individual glass pane thicknesses or glass pane spacing in a multiple pane insulated glass window.
It also is known in the art that glass pane thickness and the spacings between multiple panes of glass may be measured by directing a laser beam at a known angle, for example, at an angle of about 45.degree., to a surface of the glass product. A portion of the laser beam is reflected and a portion is refracted at each glass surface. The thickness of the glass layers and of any spacing between the layers is determined by impinging the reflections on a surface and measuring the spacings on the surface between the reflections. Different scales are required for measuring the glass thickness and for measuring the spacing between panes of glass due to differing indexes of refraction at the surfaces between glass and gas.
In one known gauge for measuring glass pane thickness and spacing, the laser source is mounted in a housing which is positioned against a glass surface during a measurement. A laser beam is directed at an angle to the surface and the beam reflections impinge on a scale on a slide which is moveable in a slot in the housing for adjustment towards and away from the laser source in a plane parallel to the glass surface. The scale slide may be adjusted to align a zero reference mark with a first reflection from the glass surface which the housing abuts. Additional reflections show on the scale for each additional glass surface interface. The reflections will occur at interfaces between glass and gas, and where layers of glass are laminated together. Separate slides with separate scales are provided for measuring glass plate thickness and for measuring the spacing between panes of glass due to differences in refraction of the laser beam between glass and gas.
In a modified prior art gauge for measuring glass pane thickness and glass pane spacing, a slide is provided with a central section on which the reflected laser beam impinges. Different scales are provided on opposite sides of the central section, one for measuring glass thickness and one for measuring glass pane spacings. The slide moves in a dovetail slot and is spring loaded to eliminate looseness in the slot. For strength, the slide has a thickness of about one eighth inch (about 32 mm) or more. In order to clearly see the reflected laser beam, it is necessary for the user to position his or her eye substantially in line with the reflected beam. If the laser beam is directed at an angle of 45.degree. to the glass, then the eye also must be positioned along a line at an angle of about 45.degree. to the gauge in order to clearly see the reflections. In bright light, it can be difficult to read the gauge. Further, multiple reflections tended to occur at both surfaces of the slide. These reflections can lead to false measurements.
In still another prior art gauge for measuring glass thickness and glass pane spacing, reflections of a laser beam from the glass-to-gas interfaces impinge on a photosensitive array. A microprocessor determines the locations of the reflections and provides a digital readout of the thickness of each glass pane and of the spacings of multiple glass panes. This gauge is considerably more expensive than a gauge in which the thickness and spacing measurements are read directly from reflections on scales.
It is desirable to provide a low cost gauge for measuring glass thickness and glass pane spacing which is easy to use and to read and in which false readings from internal reflections in the slide are not present.